I want to start a new online identity and use a single common identifier on as many sites as possible. What web apps or other methods are available for me to check if my proposed nickname is already in use? I definitely want to avoid any service that might start cybersquatting on the name, if any are suspected of doing that.

Edit: agreed, this does make my online presence very searchable, and I'll give that some more thought.

This question appears to be off-topic. The users who voted to close gave this specific reason:

"Application/website recommendations are off-topic and out of scope. It is better instead to use a particular web app or website and ask for help in any issues you have with it specifically." – Alex, pnuts, Eight Days of Malaise, Al E., jonsca

Long time ago I started to use couple of random letters (Fczbkk) as my nickname. So far it was everywhere I tried to register. Even my domain (http://fczbkk.com) was free. Choosing "Fczbkk" was one of the best decisions I have ever made.

So if you just need an unique identifier for your on-line identity, I'd recommend using any random generator. Or you can smash your forehead against the keyboard or something like that.